The present invention relates to stretchable rigid films endowed of valuable mechanical properties which render the same particularly suitable for applications in the field of pallets packaging and the method of preparation thereof.
The packaging of pallet units is normally made using stretchable films or thermoshrinkable films.
The stretchable film is a multilayer film based on polyolefin resins using the cast or buble technology and can be diversified in the production stage in standard film or in prestretchable film.
The standard film is normally used for carrying out manual packaging or on semi-automatic machines subjected to light loading work.
The prestretchable film, on the contrary, is utilized for big packaging lines operating at very high production output. Actually, the user of the film applies to the platform of the pallet a film of given thickness which is suitably reduced by a particular wrapping system and film application, which stretches the same to the desired percentage (generally 100-200%) by means of two cylinders having different velocities.
It is possible in this way to limit the amount of film used assuring in the meantime an adequate mechanical holding of the load to the pallet platform.
The thermoshrinkable film can be subdivided into films with monoaxial shrinkage and films with biaxial shrinkage.
This latter one is normally used in the pallet packaging. Differently from the stretchable film, the thermoshrinkable one is applied with a more complex technique. The pallet unit is at first "hooded" with the film and then subjected to a heating process in oven at temperature higher than 100.degree. C. Thanks to that, the "hood" made with the film shrinks fixing the load to the pallet platform with a reasonable but not excessive compression stress.
The stretchable film comprises a layer of adhesive material which assures auto-adhesivity of the film during the packaging step. The films can have also sliding characteristics to avoid the sticking of the platforms when in contact.
Both films present natural limits of application.
The use of the prestretchable films is justified only in presence of automated machines with a high output: only in this case infact the cost increase and the greater complexity of the production line dictated by the presence of a suitable unit for the prestretching of the material, become acceptable.
Another inconvenience of the prestretchable film is represented in some applications by an insufficient holding force on the loads when piled in many rows, such as are realized in the industry of mineral water, paper and similar products.
For these special applications, its use is inadequate because a package is obtained lacking the mechanical resistance necessary to avoid the shifting of the load when the platform is moved.
Also the use of the thermoshrinkable film implies evident inconveniences; the running costs of the packaging process are high and in any case superior to those of the stretchable film. Furthermore it is impossible to use this product for some types of loads in view of the high temperature to which the palletized unit is subjected before wrapping.
The heating step, due to the high temperature reached, may irreversibly impair the load.